It is known in the art of long distance (i.e., greater than one mile) high data rate (i.e., greater than one GHz) optical communication systems that to preserve data transmission integrity, a narrow bandwidth (or narrow wavelength-spread) optical source should be used. More specifically, if an optical signal is digitally transmitted, i.e., coded as ones and zeros, along a optical fiber, where a zero corresponds to a low intensity beam and a one corresponds to a high intensity beam, if the wavelength-spread is too wide, ones and zeros associated with one wavelength light may get blended with ones and zeros from another wavelength. This occurs because the fiber exhibits a different refractive index (n), and consequently a different optical path length, for each wavelength. Thus, the propagation time for one wavelength will be different from that for another wavelength in the same fiber. If the data transmission rate is high, only a small amount of phase shift (or time delay) is needed to corrupt the data.
Also, it is known in the art to detect perturbations in a polarization preserving optical fiber using a broadband (or low coherence; or broad wavelength-spread) light source, a variable Michelson Interferometer, and an optical detector, as described in the articles: K. Takada et al, "Measurement Of Spatial Distribution Of Mode Coupling In Birefringent Polarization-Maintaining Fiber With New Detection Scheme", Optics Letters, Volume 11, No. 10 (October 1986) pages 680-682; M. Tsubokawa et al, "Mode Couplings Due To External Forces Distributed Along A Polarization-Maintaining Fiber: An Evaluation", Applied Optics, Volume 7, No. 1 (January 1988), pages 166-173; and K. Takada et al, "Measurement Of Spatial Distributions Of Mode Coupling In Polarization-Maintaining Fibres", Electronics Letters, Volume 20, No. 3 (February 1984), pages 119-121.
This technique provides a broadband light source which injects light into one end of a polarization preserving birefringent optical fiber transmission line. The injected light is confined to a single polarization mode, e.g., by polarizing the light along a first optical axis of the fiber. At a remote (or receiving) end the light is monitored by an optical detector after passing through an adjustable analyzing interferometer (e.g., a Michelson Interferometer having one variable-length leg). If the fiber is perturbed at some point, e.g., by a kink or a twist, the polarization of the propagating light is altered such that a small fraction of the input light in one polarization mode of the fiber is converted (or coupled) to a second polarization mode which is orthogonal to the original polarization mode. Due to the fiber birefringence, the index of refraction for each mode is different; thus, the original and orthogonal polarizations travel different effective optical distances from the perturbation to reach the remote location.
It is known that if light having a wide wavelength spread (i.e., light having many different frequencies), is split into two paths and interfered with itself, the only time the light will add coherently (i.e., all the frequencies add in-phase), is when both optical path lengths are identical. It is also known that the coherence length is an indication of how much the path lengths can differ and still have the interference be considered a coherent sum. Also, the coherence length is inversely related to the wavelength spread. For example, the wider the wavelength spread, the more frequencies that exist in the light and, thus, the smaller the path difference must be to achieve a coherent sum. For example, a one nanometer bandwidth yields a coherence length of about 2 millimeters.
If the wavelength spread of the light source is broad enough such that the coherence length is much shorter than the effective path difference caused by fiber birefringence, the two polarization components do not interfere coherently at the remote location. However, if these components travel unequal arm lengths of the interferometer at the remote location, they will interfere coherently when the arm length inequality equals and compensates for the path difference in the fiber (within the coherence length).
If the arm inequality is variable, and the fiber birefringent characteristics are known, the location of the fiber perturbation can be deduced from the arm inequality at which coherent interference is observed. The resolution achievable in locating the perturbation is inversely related to the probe coherence length; thus, the smaller the coherence length, the better the resolution.
It is desirable to have a high speed optical fiber data transmission system that also has the capability of detecting and accurately locating perturbations along the optical fiber simultaneously with the data transmisson. However, accurately determining the location of the perturbation, as described above, requires light with a broad wavelength spread, rather than a narrow wavelength spread needed for high speed data transmission.